As noted under Artistic License – Religion, Thorn being 1/16th Wiccan is complete nonsense. However she still clearly practices some elements of Wicca which arguably still makes her the best chance to defeat Sarah.
Also, Ben Ravencroft first heard about Mystery Inc from their solving the mystery of the "Moat Monster". The Moat Monster was the same story that Daphne brought up in her interview towards the beginning of Zombie Island, which means 'The Witch's Ghost' takes place in the same universe as 'Zombie Island'. However, Fridge Logic arises when you realize that Ravencroft wouldn't have waited however many years between Mystery Inc's initial retirement and their eventual revival just to get that specific team of kids to help him. After all, what would Ravencroft have done if the gang never got back together?
Daphne and Fred still operated as mystery-solvers; that is what inspired Fred to get the team back together in Zombie Island. And they were public about it, including Daphne having her own show. All Ravencroft would have needed was to keep a watch on them, and continue his own searches in the meantime. Additionally, we could assume that he did enlist the "help" of other mystery-solving gangs...if we also assume that there are in fact other mystery-solving gangs.